Stretched films such as polyvinyl chloride films, polypropylene films and polyethylene films are hitherto known as heat shrinkable films.
Among them, heat shrinkable polypropylene films have been generally used in high speed automatic packaging, since they have a good film firmess and are excellent in packaging machine applicability. However, as compared with heat shrinkable polyvinyl chloride films, the heat shrinkable polypropylene films exhibit heat sealability and heat shrinkability only at fairly high temperatures. Moreover, the propagation tear resistance and impact strength of the film are low, so it cannot necessarily be said that the heat shrinkable polypropylene films have a good applicability to packaging of heavy weight articles and the like.
The above-mentioned heat shrinkable polypropylene films are the heat shrinkable films made of crystalline isotactic polypropylene which is an already known material, copolymers composed mainly of polypropylene such as propylene-ethylene copolymer and propylene-ethylene-butene terpolymer, and mixtures of these polymers with thermoplastic polymers capable of forming a film when mixed with these polymers.
In order to eliminate the above-mentioned defects, particularly to improve the heat sealability and the propagation tear resistance, packaging films using a syndiotactic polypropylene in the surface layers are reported in Japanese Patent Publication Kokai Nos. 5-77309, 5-200957 and 5-245992.
However, though these proposals improve the heat sealability and the propagation tear resistance, it cannot be said that the low temperature shrinkability is satisfactory. Also, since the ratio of the syndiotactic polypropylene layers to the whole layers is small, the effect on the propagation tear resistance is small. Further, because the syndiotactic polypropylene having a low melting point is used in the surface layers, the films have a defect of whitening if a package is shrinked at high temperatures.
The present invention is intended to provide a film constitution which improves the low temperature heat shrinkability and the propagation tear resistance without deteriorating the packaging machine applicability and optical property that conventional heat shrinkable films using a polypropylene resin possess, and moreover which has a wide range of heat shrinking temperature.